1. Field of the Invention
The present invention relates generally to materials testing and, more specifically, to non-destructive testing for temper embrittlement in metals based on measurements of magneto-acoustic emission, magnetic remanence, and magneto-acoustic remanence.
2. Description of the Related Art
Temper embrittlement is the losso of impact toughness which occurs in susceptible alloy steel when heated within or slowly cooled through the temperature range of approximately 371.degree. to 599.degree. C. (700.degree. to 1110.degree. F.). The problem of temper embrittlement is caused by migration of impurity elements such as sulfur, phosphorus, tin, antimony, and arsenic to prior austenite grain boundaries. It has been known that although these embrittling elements are typically present in bulk weight percentage concentrations of 20 to 200 ppm, grain boundaries of embrittled steel can contain concentrations that are one or two orders of magnitude greater due to segregation during heat treatment.
Temper embrittlement does not occur in high purity steels. Unfortunately, the manufacture of high purity alloy steels is cost prohibitive. These high purity steels are used primarily when research grade materials are needed. Commercial grade steels, such as commercial grade HY80, which is a type of Ni-Cr-Mo-V alloy casting steel, contain various impurity elements which can cause temper embrittlement. Steels such as commercial grade HY80 are in extensive use today due to their high yield strength and high impact toughness. However, these properties can be degraded over time due to exposure to high temperatures. Temper embrittlement, which may be present in the material initially, or may develop over time, remains latent until a component made from the material fractures. Fracture of components used in the aerospace industry and elsewhere can result in loss of life or property, or both.
Presently, there are a number of testing techniques for determining the presence of temper embrittlement in metal components. Many commonly used testing procedures involve destructive mechanical testing, such as the well known Charpy V-Notch (CVN) test. In this test, a V-notch is cut into the specimen, and a sample is placed in an impacting device that records the amount of energy necessary to break the sample. The procedure is an expensive one which leaves a hole in the component. The hole must be filled with weld material after testing.
Non-destructive testing techniques are preferred for obvious reasons. One such non-destructive method is described in U.S. Pat. No. 4,408,160. There, it was recognized that a variable magnetic field results in the movement of domain walls which creates an acoustic wave known as the acoustic Barkhausen signal. The signal was found to vary in accordance with stress. A variable magnetic field was applied to a specimen so that, as the field varies, an acoustic vibratory wave was picked up by a transducer. The signal was amplified and filtered and then analyzed to determine pulse widths in a certain range which are characteristic of stress.
U.S. Pat. No. 4,692,701 describes a method of testing for temper embrittlement in steam turbine rotors. A coil was used to magnetize the specimen and produce a Barkhausen magnetic signal. The signals were compared to those of a non-embrittled rotor and an observable variance between the two determined specimen embrittlement.
U.S. Pat. No. 4,689,558 also describes a non-destructive testing technique which relies on the Barkhausen effect. The invention focuses on the determination of a fatigue limit of the specimen, which limit coincides with a maximum width root-mean-square (rms) value of the frequency of the Barkhausen signal.